I did a girls' weekend with some friends in Reykjavik a few years ago and we had a blast. The hotel we stayed at (the Nordica) was nice - very crisp and modern, with a great restaurant and spa - I think I had the best massage of my life. We did the hot springs at the Blue Lagoon and some spa treatments there as well. We also spent a day driving around visiting some of the geysers and waterfalls, and walking around the city - the whole area is very manageable both walking and driving, and is beautiful. And if they're interested in nightlife, it's excellent! The only downside is that it can be expensive - we got a very cheap package deal with our airfare, hotel, rental car and breakfasts included from Icelandair so our overall costs were very reasonable, but food and drink were pricey.